The invention relates to the field of the optical detection of objects and more specifically relates to an extensive field camera, based on the real time generation of a complex wave front, which strengthens the wave front coming from the object to be detected.
In general terms, such a camera comprises an optical system and a radiation detector. The optical system forms an image of the object on a detector, which can restore it directly, e.g. in the case of photographic emulsions, or indirectly via electrical signals, e.g. in the case of television. In the case of objects which are not in themselves luminous and which are not naturally illuminated object illumination means are added to the camera.
In the case of remote objects, it is advantageous to use infrared radiation, which is absorbed less by the ambient medium than visible radiation. However, in the infrared range, detectors have a relatively limited sensitivity.
In order to improve the detection sensitivity, attempts have been made to take advantage of the interference phenomenon between two coherent light waves and the development of the laser has made it possible to produce detection equipment based on this phenomenon.
In the case of plane waves, the detection signal-to-noise ratio is improved by heterodyning. This process consists of making the object wave front interfere with a reference wave serving as a coherent local oscillator, but which has a limited frequency shift compared with the object wave. The two plane wave surfaces made colinear via e.g. a semitransparent plate reach the square-law, opto-electrical detector either directly or via a collecting optical system and they then interfere in a coherent manner. In the case of parallel polarizations of the two waves on leaving the detector, the component at the difference frequency between the frequencies of the object wave and the reference wave has an amplitude which is proportional to the square root of the product of the optical powers of these two waves.
One of the main interests of this type of detection is the gain obtained with respect to the signal-to-noise ratio on working with a reference wave, whose power is well above that of the object wave. This gain compared with direct detection is equal to the ratio of the powers of the reference wave and the object wave.
However, this process has the disadvantage of giving rise to a detection in a field angle which is only open to a limited extent. The photocurrent emitted by the opto-electrical detector decreases very rapidly with the angle which can be formed between the object wave and the wave serving as the local oscillator. The attenuation of the signal with this angular separation is dependent on the wavelength, the diameter of the sensitive face of the detector and the aperture of the collecting optical system. For example, for a wavelength of 10 microns and a 10 cm diameter collector lens, a signal drop of 3 dB is obtained for an angular separation of 10.sup.-4 rad. For the formation of bi-dimensional images of objects, the number of discernable points is very limited and the process can only be used with difficulty.
This difficulty is overcome by the camera according to the invention which utilizes means for generating a wave having a complex wave front isomorphic with the object wave front to be detected.
Certain photosensitive media have the property of restoring in real time the replica of a random incident wave front and this replica can reproduce and maintain the phase of the incident wave front. By traversing the photosensitive medium and by cooperating with a reference wave or pumping wave the incident wave records there a diffraction grating. Following a recording time, the medium restores a replica of this wave containing part of the energy of the reference wave. On leaving this medium, the wave fronts of the incident wave and its replica can be exactly superimposed. Therefore, a perfect phase matching between the incident wave and its replica is obtained.